What’s behind America’s epidemic of violence? Why does the United States consistently have higher murder rates than other advanced democracies?

Liberals often blame the widespread ownership of — and easy access to — guns and ammo.

Conservatives cite a litany of reasons including violent video games, blood-spattered entertainment programming and the increasing number of single-parent families.

Both ends of the political spectrum theorize that a broken mental health care system plays a part.

That’s what we’ve heard. So … who’s right and who’s wrong?

In an attempt to make an objective analysis — by the numbers — Texas on the Potomac reviewed statistics for the U.S. and five similar countries.

Our conclusion: There is a strong statistical correlation between violence and both gun ownership and poor mental health care. There is little relationship between violence and violent video games, movies or TV shows. And there is no statistical tie between violence and single-parent households.

Taking a step back from the headline-grabbing massacres, such as Newtown, one fact is undeniable when you compare the United States to similar countries: The U.S. has consistently enjoyed a substantially higher homicide rate.

The total homicide rate was more than four times higher than the average of five similar countries (Australia, Canada, France, Germany and Great Britain), according to data from the United Nations Office on Drugs and Crime.

Even with a decrease in recent years, the US hold a substantial lead in the total homicide rate. Source: UNDOC (Graphics by Corey Kane)

Lax gun laws, the ubiquity of firearms, poor mental health care, violent media, and single parent families have all surfaced as theories to explain our exceptional violence. The Houston Chronicle compared these factors in the US with five similar countries, to see if they had any merit.

Guns per capita had the strongest correlation with homicide rates. An increase in mental health resources correlated with a drop in homicide rates. Violent media such as movies and video games showed no correlation with homicide rates, nor did rates of single parent homes.

Among the six countries, gun ownership rates had a nearly perfect correlation with homicide rates. The homicide rate includes all killings, not just firearm-related. Gun data came from the 2007 Small Arms Survey.

Correlation does not prove causation, however it can be a helpful tool in identifying likely causes. Correlation scores range from 1.0 to -1.0. Scores close to 1.0 indicate a perfect correlation, showing as one factor increases, such as number of firearms, a corresponding increase occurs in the second factor, homicides. Scores near -1.0 indicate a similar correlation, only in the opposite direction.

A correlation near zero indicates a weak to non-existent relationship between the factors.

When Congress began hearings on gun violence legislation in January, former Rep. Gabrielle Giffords, D-Ariz., urged lawmakers to act on curbing weapons. Giffords was wounded in a mass shooting in Tucson in 2011.

“Too many children are dying. Too many children. We must do something. It will be hard, but the time is now,” Giffords told Washington lawmakers, struggling to speak.

Tom Diaz, author of The Last Gun, and gun control advocate is unsurprised that gun ownership would correlate with a high homicide rate.

“They’re there,” Diaz said of guns in the U.S. “They’re available. We’re human beings.”

Diaz said many in Washington a resistant to gun control legislation, partly because advocates are not well organized, while gun rights’ activists are highly organized. Diaz said gun rights’ advocates often see the gun issue as part of a set of larger cultural issues.

“People are extremely resistant to data driven analysis,” Diaz said. “People know what they know, and they aren’t open to change.”

Mental Health

Per capita psychiatrists had a -0.70 correlation with the homicide rate. Mental health spending as a percent of total health spending had a -0.51 correlation. Source: WHO, OECD, British media

As mental health resources decreased in the six countries, homicide rates increased. The correlation was not as strong as gun ownership, but was still significant, especially with the number of psychiatrists per capita. Mental health data came from the World Health Organization, British media reports, and the Organization for Economic Cooperation and Development.

In the case of some recent mass shootings, mental health seemed to play a role. The movie theater shooter in Aurora, Colorado, is using mental illness as his defense. The 2007 Virginia Tech shooter was adjudicated mentally ill.

Republicans in Congress suggested examining the mental health system, often as an alternative to gun control, such as Iowa Senator Chuck Grassley.

“Any serious discussion of the causes of gun violence must include a complete reexamination of mental health as it relates to mass shootings,” Grassley said in January.

Ron Honberg, the legal director at the National Alliance on Mental Illness, said that treating mental illness differently than other diseases has left many bereft of treatment options.

“We know most people with serious mental illness don’t have access to mental health care,” Honberg said in a phone interview. Honberg estimated four out of 10 mentally ill people cannot get access.

Without access, many mentally ill wind up in the criminal justice system, Honberg said. He estimated that about 5 to 7 percent of the general population had mental illness, but about 15 to 20 percent of the incarcerated population were mentally ill.

Violent Media

Django Unchained sales per capita had a -0.37 correlation with the homicide rate. Video game sales per capita had a -.05 correlation. Source: Media reports on sales.

Opponents of gun control pointed to another industry as culpable: violent movies, television, and video games. The data disagree.

In a press conference shortly after Newtown, National Rifle Association president, Wayne LaPierre, made the argument.

“Then there’s the blood-soaked slasher films like American Psycho and Natural Born Killers that are aired like propaganda loops on ‘Splatterdays’ and every day, and a thousand music videos that portray life as a joke and murder as a way of life,” LaPierre said.

Media scolds have long made Quentin Tarantino, who made his Hollywood debut as the screenwriter for Natural Born Killers, the paragon of ultra-violent media profiteers. Per capita ticket sales for his latest movie, Django Unchained showed a negative correlation with homicide rates among the six countries. The movie performed better, per capita, in nations with lower rates of violence, according to media reports of ticket sales.

Another form of media came under the microscope—video games. Since the 1999 Columbine shooters many shooters were reported to have played video games, a detail not lost on Senator Grassley.

“According to media reports the perpetrators of mass shootings in Aurora and Newtown were both avid players of violent video games,” Grassley said at a March hearing on gun control.

According to media reports of video game sales in other countries, they did not correlate with homicide rates, there was almost zero positive or negative correlation.

Single Parent Families

Single parent family rates had a 0.19 correlation with the homicide rate. Source: OECD

Though given less attention, the connection between single parent families and violence did arise during the presidential election. A weak to non-existent correlation exists in the six countries, according to data from the Organization of Economic Cooperation and Development.

During the second presidential debate, a survivor of the theater shooting in Aurora, Colorado, asked what the candidates would do to address violence.

Republican candidate Mitt Romney said some of the blame should fall on a lack of two parent families.

“Because if there’s a two parent family, the prospect of living in poverty goes down dramatically,” Romney said. “The opportunities that the child will — will be able to achieve increase dramatically. So we can make changes in the way our culture works to help bring people away from violence and give them opportunity, and bring them in the American system.”

Barack Obama agreed with the connection between single parent families, poverty and violence.